The use of hydrotalcite as a component in heat stabilizing molded vinyl compound is well established, for example, as disclosed in U.S. Pat. No. 6,313,208 (Kyowa), U.S. Pat. No. 5,451,628 (Synthetic Products), and U.S. Pat. Nos. 6,413,639 and 7,405,359 (Toda Kogyo). The volume of hydrotalcite used commercially for this purpose was not significant until EU directives ROHS (Restriction Of use of Hazardous Substances), REACH (Regulation on Registration, Evaluation, Authorization and Registration) and WEEE (Waste from Electrical & Electronic Equipment) regulated the use of lead and certain components present in tin and barium-zinc heat stabilizers. This, along with the current trends in eliminating volatile organic compounds and toxic metals, primarily in architectural applications, changed the direction in vinyl stabilization towards the increased use of hydrotalcite containing heat stabilizers.
The demand for hydrotalcite containing heat stabilizers was sudden and greater than supply. Suppliers could not produce enough and the world was searched for comparable material. Expectations in vinyl processing and performance were elevated in that the new hydrotalcite containing heat stabilizers had to equal that of the well established standards. The new hydrotalcite type replacement stabilizers stabilized adequately but many difficulties with processing and performance hampered the compliance effort.
During the powder mixing of the hydrotalcite containing heat stabilizers, product densities would drop, severely. This prevented normal filling of drums, bags and sacs. Container weights dropped as much as 50% which greatly affected manufacturing productivity. Compaction of the packaged heat stabilizer occurred during normal storage, producing hard lumps in the typically free flowing powder. This increased the amount of rejected finished product. Customers complained of “rat-holing” (bridging of the powder over an orifice which restricts flow of the material) in their transfer equipment during blending of their vinyl compounds when using these hydrotalcite containing heat stabilizers. They also complained of extreme screen pack build up of sand-like material (analyzed as only hydrotalcite without the other components of the heat stabilizer or vinyl compound) during extrusion. This forced considerable down time in their plants. Finished vinyl parts displayed inconsistent rough surfaces while difficulties in obtaining UL listing on THWN and THWN-2 wire formulations incorporating the hydrotalcite containing stabilizers made for an uncertain lead stabilizer replacement program.
Many explanations were created to fit the individual difficulties. There was no understanding that these problems could be interrelated. The difficulties mentioned above diminished with some sources as well as different lots of hydrotalcite as opposed to others. Hydrotalcites used to make commercial vinyl heat stabilizers are generally based on a magnesium and aluminum metal complex or a magnesium, zinc and aluminum metal complex. The anion in both cases is preferably a carbonate, a dispersing agent may be used during reaction and the finished hydrotalcite in both are surface treated with a metal carboxylate or carboxylic acid such as stearic acid (at a level between 0.5% and 5% with the range of 1.5% to 2.5% typically used). The typical pH range is 8.0-9.5, the MgO/Al2O3 ratio ranges from around 4-5 for the magnesium and aluminum metal complex and around 3-4 for the magnesium, zinc, and aluminum metal complex. The ZnO to Al2O3 metals ratio for this complex is around 1.0. The particle size is below 6 microns and the surface area (BET) is less than 30 m2/g. The hydrotalcite can also be calcinated.
Hydrotalcite incorporated in vinyl heat stabilizers is typically represented (but with slight molar ratio variations) as follows:[M2+1-xM3+x(OH)2]x+[An−x/n,yH2O]                “Chemical Society of Japan”, 1995, no. 8, pp. 622 to 628Typically, the two different types of hydrotalcite for vinyl applications can be written as follows:Mg4Al2(OH)12CO3.3H2OandMg3.5Zn0.5Al2(OH)12CO3.3H2O        
Hydrotalcite can be made via three typical procedures such as the co-precipitation method, the salt oxides method and the induced hydroxide method. These methods have been revealed in the following U.S. patents: U.S. Pat. No. 3,879,523 Kyowa, Apr. 22, 1975; U.S. Pat. No. 4,458,026, Union Carbide, Jul. 3, 1984; U.S. Pat. No. 4,904,457, Aluminum Co. of America, Oct. 18, 1984; U.S. Pat. No. 5,399,329, Aristech Chemical Corp., Mar. 21, 1995; U.S. Pat. No. 5,578,286, Aluminum Co. of America, Nov. 26, 1996; U.S. Pat. No. 6,287,532, Kyowa, Sep. 17, 1999; U.S. Pat. Nos. 6,413,639 and 7,405,359, Toda Kogyo, Jul. 2, 2002; U.S. Pat. No. 6,514,473, Sasol, Feb. 4, 2003; U.S. Pat. No. 7,211,235, Sud Chemie AG, May 1, 2007; WO/2008/050927, Shin Won Chemical Co. LTD., Feb. 5, 2008.
A typical vinyl heat stabilizer formulated with hydrotalcite can contain alkaline earth and metal carboxylates (such as Ba, Ca, Mg, K, Li, Na, Sr, Ce, Sn, Al, Zn, La and rare earth metals), uracils, antioxidants, beta diketones and aceto acetonates, polyols, hydroxyl containing isocyanurate, alcohols, perchlorates, melamine, alkaline earth and metal oxides, alkaline earth and metal hydroxides, zeolites, silicates, and lubricants (such as paraffin, fischer tropsch and polyethylene waxes, esters, amides, C10-C30 carboxylates) among other ingredients and as described in U.S. Pat. Nos. 6,313,208, 6,413,639, and 7,405,359, to list a few. These components are powders with the process of manufacturing a commercial vinyl heat stabilizer being a simple powder blend with no intended reaction. However, as the hydrotalcite concentration within the heat stabilizer increases from about 30% to 90% by weight, higher degrees of instability and processing problems, as described above, are associated with the higher percentages of hydrotalcite used.